THIS invention relates to a system for monitoring the condition of elongate structural elements and more particularly but not exclusively, to a system for monitoring and detecting cracks and breaks in railway rails. The invention furthermore extends to the methodology of designing and developing such a system.
There are several methods and systems which have been proposed for monitoring the integrity of elongate structural elements, and in particular railway rails. These methods and systems are aimed at detecting cracks in the rails before they develop into complete breaks, and also to detect breaks in a railway network where they have already occurred. If a crack or break in the rail is not detected beforehand, it could result in the derailment of the railway vehicle travelling on the track. It will be appreciated that such derailments cause financial loss and can also result in injury and loss of life. Also, it should be noted that although reference is made to railways, these systems are equally applicable to other applications where lengths of structural steel are utilised, such as for example mine shafts and bridges.
One method of detecting cracks and breaks in the rails of railway tracks is disclosed in South African patent 99/6936, the contents of which is incorporated herein by reference. The method includes the step of providing a number of autonomous acoustic transmitter units, and a number of acoustic receiver units located between the transmitter units. The various units are spaced apart from one another by predetermined distances. The transmitter units introduce a series of acoustic pulses with specific frequency composition into the rails and the receiver units detect and analyse the pulses to monitor any unwanted condition concerning the rail. This method requires the use of transmitters and the use of receivers in order to monitor the condition of the rail.
Development of transducers for this method of detecting and monitoring cracks and breaks in railway rails is discussed in “Development of piezoelectric transducers for a railway integrity monitoring system”, Philip W, Loveday, Smart Structures and Materials 2000: Smart Systems for Bridges, Structures, and Highways, Proceedings of SPIE Vol. 3988, 2000, Newport Beach, pp. 330-338. The system makes use of piezoelectric transducers which are mounted (clamped) under the crown of the rail on the outside of the track. The method of clamping the piezoelectric transducers is described in PCT patent application WO 2004/098974, the content of which is incorporated herein by reference.
The piezoelectric transducers are spaced along the length of the railway network and they periodically transmit ultrasonic waves through the rails. The waves propagate through the track from one transducer towards a downstream transducer which acts as a receiving station. Typically, the transducers are spaced about 1 km apart. If the ultrasonic signal is not detected at the receiver station, the receiver station activates an alarm indicating that the rail either has a crack or is broken.
A disadvantage associated with the above system is that the piezoelectric transducers are attached (clamped) under the crown of the rail on the outside of the track. The piezoelectric transducers are large and cannot be attached under the crown on the inside of the track because they would interfere with the train wheels. The piezoelectric transducers have to be removed from the rail during routine track maintenance because a ‘tamping’ machine used to re-pack the ballast under the sleepers has wheels that engage the outside of the crown. The removal and re-attachment (which requires re-tightening of the clamps two weeks after re-attachment) of the piezoelectric transducers increases the maintenance cost of the system and results in periods of time when the system is inoperable.
In addition, the existing system is not suited for distance in excess of 1 km, as the transmitted signal is not strong enough, and because the transducer is also not accurately matched to the particular structural element to which it will be attached from a propagation and operating frequency point of view.
The detection systems described above have generally been developed using design methodologies that do not optimally incorporate the use of mathematical modelling techniques in which the transducer and rail response is mathematically modelled, and in which the transducer is then designed in an iterative manner. This resulted in the selection of transducers that are not necessarily optimized for a particular application, and which may result in the transducers being larger than required in practice, whilst also not performing optimally insofar as transmission and receiving of signals are concerned.
It is therefore an object of the invention to provide a system for monitoring and detecting cracks and breaks in railway rails that will address the disadvantages described above.
It is also an object of the invention to provide a piezoelectric transducer for use in the system according to the present invention.
It is a further object of the invention to provide a method for developing a transducer-based failure detection system, which will at least partially overcome the above disadvantages, and which will also be a novel and useful alternative to existing design methodologies.